Generation of multiple low-noise copies of optical signals

ABSTRACT

A system and method for generating copies of a signal includes a first radiation source configured for providing a plurality of pump radiation beams, a second radiation source configured for providing a signal radiation beam, and a second-order nonlinear optical medium to receive the plurality of pump radiation beams from the first radiation source and the signal radiation beam from the second radiation source and to emit a plurality of idlers, where the plurality of idlers are low-noise copies of the signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/251,450 filed Nov. 5, 2015, entitled, “GENERATION OF MULTIPLELOW-NOISE COPIES OF OPTICAL SIGNALS” which is incorporated herein byreference in its entirety.

FIELD OF INVENTION

The present invention relates to signal processing and specifically,generating multiple faithful copies of a signal for multicasting.

BACKGROUND OF INVENTION

Multicasting is a useful signal-processing function in which copies of asignal, i.e., idlers, can be broadcast to multiple users. Multiplecopies of a given signal may also be used to sampleshort/broad-bandwidth signals in both the time and frequency domains,the former is called waveform sampling and the latter is calledchannelization, respectively. The efficacy of these techniques isdependent the quality of the copies of the signal. Ideally, idlersshould have the same shapes/spectra as the signal, and should not bepolluted with excess noise, which makes them harder to read (i.e.,measure).

SUMMARY OF INVENTION

Shortcomings of the prior art are overcome and additional advantages areprovided through a system for generating copies of a signal. In oneembodiment, the system includes a first radiation source configured forproviding a plurality of pump radiation beams; a second radiation sourceconfigured for providing a signal radiation beam, and a second-ordernonlinear optical medium to receive the plurality of pump radiationbeams from the first radiation source and the signal radiation beam fromthe second radiation source and to emit a plurality of idlers, whereinthe plurality of idlers comprise low-noise copies of the signal.

Shortcomings of the prior art are overcome and additional advantages areprovided through method of generating copies of a signal. In oneembodiment, the method includes obtaining, by a second-order nonlinearoptical medium, a plurality of pump radiation beams from a firstradiation source; obtaining, by the second-order nonlinear opticalmedium, a signal radiation beam from a second radiation source; andemitting, from the second-order nonlinear optical medium, a plurality ofidlers, wherein the plurality of idlers comprise low-noise copies of thesignal.

Additional features are realized through the techniques of the presentinvention. Other embodiments and aspects of the invention are describedin detail herein and are considered a part of the claimed invention.

BRIEF DESCRIPTION OF DRAWINGS

One or more aspects of the present invention are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and objects, features, andadvantages of one or more aspects of the invention are apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings.

FIG. 1 depicts certain aspects of an embodiment of the presentinvention.

FIG. 2 depicts certain aspects of an embodiment of the presentinvention.

FIG. 3 depicts certain aspects of an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Aspects of the present invention and certain features, advantages, anddetails thereof, are explained more fully below with reference to thenon-limiting examples illustrated in the accompanying drawings.Descriptions of well-known materials, fabrication tools, processingtechniques, etc., are omitted so as not to unnecessarily obscure theinvention in detail. It should be understood, however, that the detaileddescription and the specific examples, while indicating aspects of theinvention, are given by way of illustration only, and not by way oflimitation. Various substitutions, modifications, additions, and/orarrangements, within the spirit and/or scope of the underlying inventiveconcepts will be apparent to those skilled in the art from thisdisclosure.

Aspects of the present invention provide systems and methods forgenerating multiple low-noise, yet high quality, copies of an opticalsignal utilizing three-wave mixing (TWM), and in particular,difference-frequency generation (DFG) processes. In an embodiment of thepresent invention, three-wave mixing processes occur in second-order(chi2) nonlinear optical media. An embodiment of the present inventionincludes DFG driven by multiple pumps, which amplifies a signal andgenerates multiple idlers (copies of the signal); one idler is generatedper each pump. In various embodiments of the present invention, thesepumps can be produced by individual lasers, and/or an optical frequencycomb. The quality of an optical signal (or idler) is quantified by itssignal-to-noise ratio (SNR), which is the coherent signal power dividedby the noise power. The noise figure of idler generation (copying) isthe SNR of the input signal divided by the SNR of the output idler SNR.It is a figure of demerit. Noise figures cannot be less than 1 (0 dB)and noise figures of order 1 are considered good. In an embodiment ofthe present invention, in the high-gain regime, each idler noise figureis 2 (3 dB), independent of the number of idlers. Thus, by utilizingDFG, an embodiment of the present invention can generate a large numberof idlers and regardless of this number, experience the same noise, asnoise is independent of the number of idlers. In this manner,embodiments of the present invention can be utilized to generatemultiple copies of an optical signal without each subsequent copyfurther degenerating the quality of the copies overall.

Multiple-pump DFG is mathematically equivalent to single-pump DFGfollowed by multiple-mode idler beam-splitting. When utilizing DFG in anembodiment of the present invention, in single-pump DFG (which has anoise figure of 3 dB) the idler-sum mode has a strong coherent componentand strong fluctuations. Such a mode is insensitive to the vacuumfluctuations associated with the virtual beam splitter. The physicalidler modes produced by the virtual bean splitter also have strongcoherent fields and strong fluctuations, which are correlated. Beamsplitting reduces the coherent and incoherent powers by the same amount,so each physical idler mode has the same SNR as the idler sum-mode. Inan embodiment of the present invention, parametric amplification by DFGadds excess noise to an output signal and idlers, so no output is aperfect copy of the signal. However, as understood by one of skill inthe art, DFG produces outputs that are amplified copies of the signalwhose fluctuations are correlated and phase-dependent.

Aspects of the present invention provide certain advantages over methodpresently utilized to create multiple copies of optical signals. Somemulticasters presently utilize four-wave mixing (FWM) processes, whichunlike the processes of embodiments of the present invention, are hardto model. One advantage of utilizing three-wave mixing (TWM), inaccordance with certain embodiments of the present invention, is thatTWM processes (which occur in different media), when utilized in theseembodiments, can provide the same functionality as FWM multicasters, butthere are fewer sideband interactions in a TWM-based multicaster, makingfor more faithful copies of the signal. Another advantage of embodimentsof the present invention is that the present method enables the creationof an unlimited number of copies of a signal, without significantdeterioration of the signal, regardless of the number of copies. Incontrast, in FWM methods, in most cases, the noise figure of eachidler-generation processes scales linearly with the number of idlers, somaking more copies usually results in lower-quality idlers. Anotheradvantage of certain embodiments of the present invention that theyenable optical preprocessing to replace high-speed electrical samplingby multiple instances of moderate-speed sampling and broad-bandwidthelectrical sampling by multiple instances of moderate-bandwidthsampling. Applications of these sampling schemes include theanalog-to-digital conversion and channelization of radio-frequencysignals.

Historically, DFG (and sum-frequency generation, SFG) were implementedwith one pump, and one idler (copy) was produced. Embodiments of thepresent invention utilize multiple pumps to generate multiple idlers.Certain embodiments of the present invention involve a moderately-noisyDFG process that produces more useable multiple idlers than a noiselessSFG process.

The utilization of DFG in an embodiment of the present invention isexplained in detailed in the paper Signal replication by multiple sum-or difference frequency generation, which is provided as Appendix A andincorporated herein in its entirety.

FIG. 1 is a frequency diagram depicting DFG driven by one pump 110 andthree pumps 120, in accordance with an embodiment of the presentinvention. In TWM, which occurs in a second-order nonlinear medium, astrong pump wave drives weak signal and idler waves. The longer arrowsdenote the pumps 130 a-130 d, the arrows to the left denote the signal140 a-140 b, and the most central arrows denote the idlers 150 a-150 d.As noted above, aspects of the present invention produce one idler foreach pump. In DFG, the signal is amplified and an idler is generated,whereas in sum-frequency generation (SFG), another type of TWM describedin the Appendix, the signal power is transferred to the idler. If thesingle pump is replaced by multiple pumps (e.g., a frequency comb),multiple TWM processes occur. Embodiments of the present inventionutilize DFG instead of SFG because while the common noise figure in SFGis tied to the number of pumps, in DFG, the common noise figure isindependent of the number of pumps, enabling the generation of manylow-noise idlers. Each DFG idler is a conjugated copy of the signal.

FIG. 2 also depicts aspects of an embodiment of the present invention.In FIG. 2, pump 210 (p), signal 220 (s), and idler 230 (i) comprise,respectively, pump, signal and idler laser beams. SONM 240 is asecond-order nonlinear optical medium (material). Beams enter on theleft and exit on the right. Extrapolating FIG. 1 to FIG. 2, byincreasing the number of pumps, the number of idlers will also increase.As seen in FIG. 2, beams enter the SONM 240 (e.g., a medium) on the leftand exit on the right. As can be seen in FIG. 2, the SONM 240 is notneeded to produce the pump(s) and signal. In an embodiment of thepresent invention, the pump(s) and the signal originate from lasers, butthe idlers are generated by the SONM 240.

In an embodiment of the present invention, DFG produces outputs that areamplified copies of the signal and whose fluctuations are correlated andphase-dependent. The correlations are maximal when φ_(i)=−φ_(s) whichcase each local-oscillator (LO) phase is aligned with the appropriateoutput phase. (The real fluctuations are correlated, whereas theimaginary ones are anti-correlated.) In the high-gain regime (μ≈ν>1),the output fluctuations are correlated completely. The output idler is aperfect (conjugated) copy of the output signal. By combining thepreceding results, one obtains the noise figures in Equation 1 below. Inthe high-gain regime, the signal and idler noise figures areapproximately 2 (3 dB), in this non-limiting example. Parametricamplification by DFG adds excess noise to the output signal and idler(so neither output is a perfect copy of the input signal).

F _(s)=1+ν²/μ² ,F _(s)=1+μ²/ν².  (Equation 1)

In an embodiment of the present invention, the (common) idler-generationnoise figure is 2 (3 dB), independent of the number of pumps. Thisquality of embodiments of the present invention can be demonstratedutilizing the coupled-mode equations (CMEs) for DFG driven by multiplepumps. The solutions (transfer functions) can be used to determine acovariance matrix (quadrature variances and correlations) of the outputsignal and idlers, and the noise figures of the idler-generationprocesses. This mathematical demonstrations of the results can beunderstood in terms of physical embodiments of the present invention interms of superposition modes and simpler processes. In an embodiment ofthe present invention, beam splitting does not reduce the SNRs of theoutput modes relative to that of the input sum mode, because theamplified fluctuations associated with the sum mode swamp the vacuumfluctuations associated with the others modes.

In an embodiment of the present invention, in DFG (which has a noisefigure of 3 dB) the idler-sum mode has a strong coherent field andstrong fluctuations. Such a mode is insensitive to the vacuumfluctuations associated with the virtual beam splitter. The physicalmodes also have strong coherent fields and strong fluctuations, whichare correlated. Beam splitting reduces the coherent and incoherentpowers by the same factor of n, so each physical mode has the same SNRas the sum mode.

An embodiment of the present invention includes a system and a methodfor generating copies of a signal. In one aspect, the system may includea first radiation source configured for providing a plurality of pumpradiation beams and a second radiation source configured for providing asignal radiation beam. The system may also include a second-ordernonlinear optical medium to receive the plurality of pump radiationbeams from the first radiation source and the signal radiation beam fromthe second radiation source and to emit a plurality of idlers, which asaforementioned, are low-noise copies of the signal.

In another aspect of an embodiment of the present invention, in thesystem and/or method, an idler-generation noise figure is independent ofthe number of pumps.

In an embodiment of the present invention, the first radiation sourcemay include an optical frequency comb.

In an embodiment of the present invention, the number of pumps may equalthe number of idlers wherein the plurality of pump radiation beams isequal to the plurality of idlers.

In an embodiment of the present invention, the first radiation sourcemay include at least two individual lasers.

In an embodiment of the present invention, the idlers may includeamplified copies of the signal radiation beam and the idlers may becorrelated and phase-dependent.

An embodiment of the present invention may include an opticalpreprocessing device to receive the idlers emitted from the medium. Thispre-processing device may utilize a portion of the idlers in one of:analog-to-digital conversion, or channelization of radio-frequencysignals.

An example and further explanation of certain aspects of an embodimentof the present invention is discussed below.

As aformentioned, in FWM, which occurs in a third-order nonlinear medium(such as a fiber), one or two strong pump waves drive weak signal andidler waves (sidebands). Thus, parametric devices based on FWM canamplify, frequency-convert and phase-conjugate optical signals incommunication systems. They also enable high performance sampling, inboth the time and frequency domains. When a train of similar signalpulses is illuminated by a train of short pump pulses, each pump pulsegenerates a short idler pulse (by degenerate FWM), whose peak power isproportional to the signal power at the instant it was sampled (andwhose energy can be measured by a moderate-speed detector). If theseparation of the pump pulses differs slightly from that of the signals,each pump samples a different time-slice of its signal, so the averageshape (waveform) of the signal pulses can be inferred. However, idmultiple copies of the signal are made, each with a different carrierfrequency, then, by passing the copies (idlers) through a dispersivemedium, one can delay them relative to one another, so that a short pumppulse illuminates different time-slices of the idlers. In this way, onecan measure the shape of an individual signal pulse. Alternatively, onecan send the idlers through a periodic frequency filter. If the spacingbetween the passband frequencies differs slightly from that of the idlerfrequencies, each passband transmits (samples) a different part of itsidler spectrum. In this way, one can measure the complete spectrum of abroad-bandwidth signal or separate the spectra of synchronousnarrow-bandwidth signals. Thus, one can use optical preprocessing toreplace high-speed electrical sampling by multiple instances ofmoderate-speed sampling and broad-bandwidth electrical sampling bymultiple instances of moderate-bandwidth sampling. As discussed above,applications of these sampling schemes include the analog-to-digitalconversion and channelization of radio-frequency signals.

The schemes discussed above are enabled by the generation of faithfulcopies of the signal: The idlers should have the same shapes (spectra)as the signal, and should not be polluted with excess noise, which makesthem harder to read (measure) than the signal. In the standard copying(replication) scheme, two strong pumps and a weak signal are launchedinto a fiber. Multiple FWM processes produce new pumps and idlers (whichare copies of the signal). Although this scheme works well (and producesoutputs that are amplified versions of the input signal), it also hasdrawbacks. First, the number of FWM processes increases faster than thesquare of the number of pumps or sidebands, so it is difficult to modelreplication analytically. Strategies for optimizing the operation of aFWM-based copier (for example, equalizing the output idler powers) mustbe determined by doing numerical simulations based on the nonlinearSchrodinger equation and using numerical search algorithms. Second, inmultiple-sideband mixing the noise figure of each idler-generationprocess usually scales linearly with the number of sidebands, so makingmore copies usually results in lower-quality idlers. Fortunately,numerical simulations revealed specific dispersion conditions underwhich the idler noise figures can be limited to about 6 dB. This limitis 3-dB higher than the noise figure of two-sideband amplification and6-dB higher than the noise figure of two-sideband frequency conversion.Because of the aforementioned issues, it is useful to consider otherreplication schemes.

In three-wave mixing (TWM), which occurs in a second-order nonlinearmedium, a strong pump wave drives weak signal and idler waves. In DFG,the signal is amplified and an idler is generated (π_(p)->π_(s)+π_(i)where π_(j) represents a photon with frequency ω_(j)). The letter “p”represents pump, “s” represents signal, and “i” represents idler. Asseen in FIG. 1, each DFG idler is a conjugated copy of the signal.

DFG can be utilized to process a strong pump wave, which does not evolveand drive weak signal and idler waves (modes), which do evolve. In oneexample, α and α_(i) are the signal and idler mode amplitude operators,respectively. These operators satisfy the boson commutation relation[α_(j), α_(k)]=0 and [α_(j), α_(k) ^(↑)]=δ_(jk), where [x, y]=xy−yx is acommutator, ↑ is a Hermitian conjugate and δ_(jk) is the Kroneckerdelta. The real and imaginary parts of the mode amplitudes (quadratures)can be measured by homodyne detection, which involves a local oscillator(LO). Each quadrature operator can be represented Equation 2, whereφ_(j) is an LO phase.

p _(j)(φ_(j))=(a _(j) ^(↑) e ^(iφ) ^(j) +a _(j) e ^(−iφ) ^(j))/2^(1/2)  (Equation 2)

By combining this definition with the amplitude commutation relations,one obtains the quadrature commutation relations, noted in Equation 3.

[p _(j)(φ_(j)),p _(k)(φ_(k)+π/2)]=iδ _(jk),  (Equation 3)

The input signal and idler are characterized by their quadrature means,

p_(j)(φ_(j))

, where

is an expectation value, and the quadrature covariance matrix is notedbelow as Equation 4.

C _(p)(φ_(j),φ_(k))=

δp _(j)(φ_(j))δp _(k)(φ_(k))

, where δp _(j)(φ_(j))=p _(j)(φ_(j))−

p _(j)(φ_(j))

   (Equation 4)

For a coherent-state (CS) signal and a vacuum-state (VS) idler, thequadrature means is represented by Equation 5 below.

p _(s)(φ_(s))

=2^(1/2)|α|cos(φ_(s)−φ_(α)),

p _(i)(φ_(i))

=0,  (Equation 5)

In Equation 5, α=|α|e^(iφ) ^(α) the CS amplitude. The measured signalquadrature is maximal when the LO phase equals the signal phase,(φ_(s)=φ_(α)).

A covariance matrix is Equation 6 below, where σ=½. Equation 6 showsthat the input signal and idler have uncorrelated vacuum-levelfluctuations, which do not depend on the LO phases. For reference, thesignal-to-noise ratio (SNR) of the signal is defined to be the square ofthe quadrature mean divided by the quadrature variance. The SNR attainsits maximal value 4|α|² when the LO phase equals the signal phase.

$\begin{matrix}{{C_{p}\left( {\varphi_{s},\varphi_{i}} \right)} = {\sigma \begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

In the high-conversion regime (p≈1), the idler noise figure isapproximately 1 (0 dB), which means that the output idler is a perfectcopy of the input signal. DFG is governed by the IO equations,reproduced below, which involve the signal and idler operators, andtheir conjugates. The transfer coefficients μ and ν satisfy theauxiliary equation |μ|²−|ν|²=1, which ensures that signal and idlerphotons are produced in pairs. By choosing the phase references of theinput and output modes judiciously, one can replace Equation 7 bysimilar equations, in which μ and ν are real. By combining Equation 7with their conjugates, one obtains the quadrature I0 equations, Equation8 and Equation 9, reproduced with Equation 7 below, and with Equation10, which is the output means of the IUO quadrature equations, Equation8 and Equation 9.

b _(s)=μα_(s)+να_(i) ⁺ ,b _(i)=να_(s) ⁺+μα_(i),  (Equation 7)

q _(s)(φ_(s))=μp _(s)(φ_(s))+νp _(i)(−φ_(s))  (Equation 8)

q _(i)(φ_(i))=νp _(s)(−φ_(i))+μp _(i)(φ_(i))(Equation 9)

q _(s)(φ_(s))

=μ

p _(s)(φ_(s))

,

q _(i)(φ_(i))

=ν

p _(s)(−φ_(i))

  (Equation 10)

The output signal quadrature is proportional to the input signalquadrature, measured with the same LO phase, whereas the output idlerquadrature is proportional to the input signal quadrature, measured withthe opposite LO phase. This opposite-phase relation is a consequence ofthe conjugates in Equation 7, and signifies that the output idler is aconjugated copy of the input signal. As understood by one of skill inthe art, if one were to derive IO equations for the real and imaginaryquadratures separately, one would find that the real equations areidentical to the preceding ones, whereas the imaginary equations differonly in the sign of ν. The output covariance matrix is included below asEquation 11.

$\begin{matrix}{{C_{q}\left( {\varphi_{s},\varphi_{i}} \right)} = {\sigma \begin{bmatrix}\left( {\mu^{2} + v^{2}} \right) & {2\mu \; v\; {\cos \left( {\varphi_{s} + \varphi_{i}} \right)}} \\{2\mu \; v\; {\cos \left( {\varphi_{i} + \varphi_{s}} \right)}} & \left( {\mu^{2} + v^{2}} \right)\end{bmatrix}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

Thus, DFG produces outputs that are amplified copies of the signal, andwhose fluctuations are correlated and phase-dependent. The correlationsare maximimal when φ_(i)=−φ_(s), in which case each LO phase is alignedwith the appropriate output phase. Hence, the real fluctuations arecorrelated, whereas the imaginary ones are anti-correlated. In thehigh-gain regime (μ≈ν≧1), the output fluctuations are correlatedcompletely such that the output idler is a perfect (conjugated) copy ofthe output signal. By combining the preceding results, one obtains thenoise figures, as noted below in Equation 12.

F _(s)=2−1/μ² ,F _(i)=2+1/ν²  (Equation 12)

In the high-gain regime, the signal and idler noise figures areapproximately 2 (3 dB); parametric amplification by DFG adds excessnoise to the output signal and idler (so neither output is a perfectcopy of the input signal).

DFG driven by a comb of pumps (multiple DFG) is governed by the CMEs, asdemonstrated below in Equation 13.

d _(z)α_(s) =iΣ _(i)γ_(i)α_(i) ^(t) ,d _(z)α_(i) =iγ _(i)α_(s)^(t)  (Equations 13-14)

The effects of secondary TWM on DFG were discussed earlier and byproceeding as described above, the transformed CMEs are obtained. InEquations 15-16 below, γ is real, α_(s) is coupled to α_(i) ^(t) andvice versa and there is no “−” sign in the first equation. By combiningEquation 15-16 with their conjugates and using the commutationrelations, one finds Equations 17-18. By further combining Equations17-18, with each other, one obtains the Manley-Rowe-Weiss (MRW)equation, Equation 19, which is mathematical expressions utilized topredict the amount of energy in a wave that has multiple frequencies.The MRW equation, Equation 19, reflects that signal and idler photonsare produced in pairs (e.g., π_(s) and π_(j), or π_(s) and π₂, etc.). Bydefining a pseudo-amplitude vector, X=[α_(s) ^(t), α₁, . . . α_(n)]^(t),the general solution of Equations 13-14 can be written in matrix IOform, as Equation 20 below, with the transfer matrix of Equation 21.

$\begin{matrix}{{{d_{z}a_{s}} = {\gamma {\sum_{i}a_{i}^{\dagger}}}},{{d_{z}a_{i}} = {\gamma \; a_{s}^{\dagger}}}} & \; \\{{{d_{z}a_{s}^{\dagger}a_{s}} = {\gamma {\sum_{i}\left( {{a_{s}^{\dagger}a_{i}^{\dagger}} + {a_{s}a_{i}}} \right)}}},{{d_{z}a_{i}^{\dagger}a_{i}} = {\gamma \left( {{a_{s}^{\dagger}a_{i}^{\dagger}} + {a_{s}a_{i}}} \right)}}} & \left( {{Equation}\mspace{14mu} 17\text{-}18} \right) \\{{d_{z}\left( {{a_{s}^{\dagger}a_{s}} - {\sum_{i}{a_{i}^{\dagger}a_{i}}}} \right)} = 0} & \left( {{Equation}\mspace{14mu} 19} \right) \\{{X(z)} = {{T(z)}{X(0)}}} & \left( {{Equation}\mspace{14mu} 20} \right) \\{{T(z)} = \begin{bmatrix}c & s^{\prime} & \ldots & s^{\prime} \\s^{\prime} & {1 + c^{\prime}} & \ldots & c^{\prime} \\\vdots & \vdots & \ddots & \vdots \\s^{\prime} & c^{\prime} & \ldots & {1 + c^{\prime}}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 21} \right)\end{matrix}$

(Equation 15-16)

Equations 22-25 are true in the above expressions.

c=cos h(γn ^(1/2) z),c ^(t)=(c−1)/n  (Equation 22)

s ^(t)=sin h(γn ^(1/2) z)/n ^(1/2),  (Equation 23)

In the lower-right block of this matrix, only the diagonal elementsinclude 1s. For the symmetrical case in which α_(s)(0)≠0 and α_(i)(0)=0,the output signal power us larger than the input signal power by the(common) factor (s′)²=s²/n, where s=sin h(γn^(1/2)z). For the asymmetriccase in which α_(s)(0)=0 and α_(i)(0)=0 and α_(j)(0)=0 for j≠I, theoutput signal power is larger than the input idler power by the factors²/n. In the high-gain regime, each output idler power is larger thanthe input idler power by a factor of c²/n².

By combining the amplitude IO equations, e.g., Equation 20, with theirconjugates, the IO equations, Equations 24-25 can be obtained. Equations24-25 involve the same transfer coefficients as the amplitude equations.The imaginary quadratures satisfy similar equations, with whichτ_(sj)→−τ_(sj) and τi_(s)→−τi_(s). Thus, one can derive results for thereal quadratures, then deduce the imaginary results from the real onesby changing the sign of s′ (Equation 21). As understood by one of skillin the art, it remains true that C_(q)=TC_(p)T^(t), but it is not truethat TT^(t)=1. Through matrix multiplication, the output converse matrixbelow, Equation 26, is obtained.

$\begin{matrix}{{{q_{s}\left( \varphi_{s} \right)} = {{\tau_{ss}{p_{s}\left( \varphi_{s} \right)}} + {\sum_{j}{\tau_{sj}{p_{j}\left( {- \varphi_{s}} \right)}}}}},} & \left( {{Equation}\mspace{14mu} 24} \right) \\{{q_{i}\left( \varphi_{i} \right)} = {{\tau_{is}{p_{s}\left( {- \varphi_{i}} \right)}} + {\sum_{j}{\tau_{ij}{p_{j}\left( \varphi_{i} \right)}}}}} & \left( {{Equation}\mspace{14mu} 25} \right) \\{c_{q} = {\sigma \begin{bmatrix}{1 + {2s^{2}}} & {2{cs}^{\prime}} & \ldots & {2{cs}^{\prime}} \\{2{cs}^{\prime}} & {1 + {2\left( s^{\prime} \right)^{2}}} & \ldots & {2\left( s^{\prime} \right)^{2}} \\\vdots & \vdots & \ddots & \vdots \\{2{cs}^{\prime}} & {2\left( s^{\prime} \right)^{2}} & \ldots & {1 + \left( {2s^{\prime}} \right)^{2}}\end{bmatrix}}} & \left( {{Equation}\mspace{14mu} 26} \right)\end{matrix}$

In the lower-right block of the matrix (Equation 26), only the diagonalelements include 1s. The output fluctuations are not quantum-limited.For long distances, the output idler quadratures are correlatedcompletely. This behavior is an indication that the idlers evolve inconcert, as a sum mode. Equation 20 can be rewritten in terms of thebasis vectors. By doing so, one obtains the alternative transfer matrixwhich is labeled Equation 27 below.

$\begin{matrix}{{T^{\prime}(z)} = \begin{bmatrix}c & s & 0 & \ldots & 0 \\s & c & 0 & \ldots & 0 \\0 & 0 & 1 & \ldots & 0 \\\vdots & \vdots & \vdots & \ddots & \vdots \\0 & 0 & 0 & \ldots & 1\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 27} \right)\end{matrix}$

The alternative transfer matrix illustrates that n−1 idler-superpositionmodes are inert, and one idler-superposition mode (the sum mode)undergoes a stretching (squeezing) transformation with the signal, asdoes the (single) idler in DFG. (The sum of the signal and idler-summodes is stretched, whereas the difference between the signal andidler-sum modes is squeezed.) This is a general property of parametricprocesses with unequal numbers of amplitudes and conjugate amplitudes.This also explains why the signal- and some of the idler-generationcoefficients in Equation 21 are proportional to 1/n^(1/2). In thesymmetric case (nonzero input signal), the output signal and idler-sumpowers are larger than the input signal power by the usual factors c²and s², respectively. The sum mode is a symmetric combination of thephysical modes, so the output power in each physical mode is larger thanthe input signal power by a factor of only s²/n. In the asymmetric case,the input condition (one nonzero input idler) corresponds to anidler-sum mode with the fraction 1/n of the input idler power. Theoutput signal and idler-sum powers are larger than the input idler-sumpower by the usual factors s² and c², respectively. Hence, the outputsignal power is larger than the input idler power by the factor s²/n. Inthe high-gain regime, the contributions of the inert idler-superpositionmodes to the output idler powers are negligible. Each output idler poweris larger than the input idler-sum power by a factor of about c²/n andthe input idler power by a factor of about c²/n². One can verify thatEquation 28 below is consistent with the output covariance matrix inEquation 11.

$\begin{matrix}{C_{q}^{\prime} = \begin{bmatrix}{c^{2} + s^{2}} & {2{cs}} & 0 & \ldots & 0 \\{2{cs}} & {c^{2} + s^{2}} & 0 & \ldots & 0 \\0 & 0 & 1 & \ldots & 0 \\\vdots & \vdots & \vdots & \ddots & \vdots \\0 & 0 & 0 & \ldots & 1\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 28} \right)\end{matrix}$

As seen above, the output signal has the SNR (c(p_(s)))²/σ[1+2s²] andeach output idler has the SNR (s′(p_(s)))²/σ[1+2(s′)²]. By combiningthese results, one obtains the noise figures of Equations 29-30.

F _(s)=2−1/c ² ,F _(i)=2+1/(s′)²  (Equations 29-30)

In the high-gain regime, c²>>1 and (s′)²=s²/n>>1. The (common)idler-generation noise figure is 2, independent of n! This result can beinterpreted as idler sum-mode generation, with a noise figure of 2, asseen in Equation 12, followed by multiple-idler-mode beam splitting. Inthe present context, beam splitting does not reduce the SNRs of theoutput modes relative to that of the input sum mode, because theamplified fluctuations associated with the sum mode swamp the vacuumfluctuations associated with the other modes.

DFG driven by three pumps is illustrated in FIG. 3. As illustratedherein, secondary TWM (inverse DFG) modified the input signal andgenerates new signals with frequencies ω_(s)±ω and ω_(s)±2ω. FIG. 3 isan enhanced frequency diagram that is driven by three pumps 310 a-310 c.Also pictured are the primary and secondary signals 320 a-320 e, and theidlers 330 a-330 c.

Equation 31 is the Schmidt decomposition theorem for DFG.

$\begin{matrix}{\begin{bmatrix}B_{s} \\B_{i}^{\dagger\prime}\end{bmatrix} = {\begin{bmatrix}{V_{s}D_{\mu}U_{s}^{\dagger}} & {V_{s}D_{v}U_{i}^{\prime}} \\{V_{i}^{*}D_{v}U_{s}^{\dagger}} & {V_{i}^{*}D_{\mu}U_{i}^{t}}\end{bmatrix}\begin{bmatrix}A_{s} \\A_{i}^{\dagger\prime}\end{bmatrix}}} & \left( {{Equation}\mspace{14mu} 31} \right)\end{matrix}$

In Equation 31, D_(μ)=diag (μ_(j)), D_(ν)=diag(ν_(j)) and μ_(j) ²−ν_(j)²=1. The Schmidt coefficients, μ_(j) and ν_(j), characterize signalamplification and idler generation, respectively, and depend on theaforementioned physical parameters. In general, ν₁≧ν₂≧ . . . ≧ν_(n).Because the Schmidt coefficients depend exponentially on the pump powersand medium length, if the pump powers are sufficiently high or themedium is sufficiently long, the output is dominated by the first signaland idler Schmidt modes, and the noise figures of the associatedamplification and generation processes are 2. Once again, a challenge isto design the system in such a way that the input Schmidt mode resemblesthe physical signal mode, and the output Schmidt mode has n physicalidler components of comparable magnitude. However, DFG produces strongoutput modes with fluctuations that are stronger than vacuumfluctuations (or the fluctuations associated with the recessive modes).When these Schmidt modes are split into their physical components, thephysical noise figures depend inversely on the overlap between thephysical signal and the input Schmidt mode, but do not depend on theoverlaps between the output Schmidt mode and the physical idlers(because the coherent and incoherent parts of the output mode are splitin the same way). In particular, they do not depend on the number ofidlers. Hence, multiple DFG presents a promising scheme for signalreplication, as illustrated in FIG. 3.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise” (andany form of comprise, such as “comprises” and “comprising”), “have” (andany form of have, such as “has” and “having”), “include” (and any formof include, such as “includes” and “including”), and “contain” (and anyform contain, such as “contains” and “containing”) are open-endedlinking verbs. As a result, a method or device that “comprises”, “has”,“includes” or “contains” one or more steps or elements possesses thoseone or more steps or elements, but is not limited to possessing onlythose one or more steps or elements. Likewise, a step of a method or anelement of a device that “comprises”, “has”, “includes” or “contains”one or more features possesses those one or more features, but is notlimited to possessing only those one or more features. Furthermore, adevice or structure that is configured in a certain way is configured inat least that way, but may also be configured in ways that are notlisted.

We claim:
 1. A system for generating copies of a signal, comprising: afirst radiation source configured for providing a plurality of pumpradiation beams; a second radiation source configured for providing asignal radiation beam; and a second-order nonlinear optical medium toreceive the plurality of pump radiation beams from the first radiationsource and the signal radiation beam from the second radiation sourceand to emit a plurality of idlers, wherein the plurality of idlerscomprise low-noise copies of the signal.
 2. The system of claim 1,wherein an idler-generation noise figure is independent of a number ofthe plurality of pumps.
 3. The system of claim 1, wherein the firstradiation source comprises an optical frequency comb.
 4. The system ofclaim 1, wherein the plurality of pump radiation beams is equal to theplurality of idlers.
 5. The system of claim 1, wherein in the firstradiation source comprises at least two individual lasers.
 6. The systemof claim 1, wherein the idlers comprise amplified copies of the signalradiation beam, wherein the idlers are correlated and phase-dependent.7. The system of claim 1, further comprising: an optical preprocessingdevice to receive the plurality of idlers.
 8. The system of claim 7,wherein the optical pre-processing device utilizes a portion of theplurality of idlers in one of: analog-to-digital conversion, orchannelization of radio-frequency signals.
 9. A method of generatingcopies of a signal, comprising: obtaining, by a second-order nonlinearoptical medium, a plurality of pump radiation beams from a firstradiation source; obtaining, by the second-order nonlinear opticalmedium, a signal radiation beam from a second radiation source; andemitting, from the second-order nonlinear optical medium, a plurality ofidlers, wherein the plurality of idlers comprise low-noise copies of thesignal.
 10. The method of claim 9, wherein an idler-generation noisefigure is independent of a number of the plurality of pumps.
 11. Themethod of claim 9, wherein the first radiation source comprises anoptical frequency comb.
 12. The method of claim 9, wherein the pluralityof pump radiation beams is equal to the plurality of idlers.
 13. Themethod of claim 9, wherein in the first radiation source comprises atleast two individual lasers.
 14. The method of claim 9, wherein theidlers comprise amplified copies of the signal radiation beam, whereinthe idlers are correlated and phase-dependent.
 15. The method of claim9, further comprising: providing, by the second-order non-linear medium,to an optical preprocessing device, the plurality of idlers.
 16. Themethod of claim 15, wherein the optical pre-processing device utilizes aportion of the plurality of idlers in one of: analog-to-digitalconversion, or channelization of radio-frequency signals.
 17. A methodof assembling a system for generating copies of a signal, comprising:positioning a first radiation source configured for providing aplurality of pump radiation beams to provide the plurality of pumps to asecond-order nonlinear optical medium, wherein the plurality of pumpsenter the second-order nonlinear optical medium from a first direction;positioning a second radiation source configured for providing a signalradiation beam to provide the signal to a second-order nonlinear opticalmedium, wherein the plurality of pumps enter the second-order nonlinearoptical medium from the first direction; and positioning thesecond-order nonlinear optical medium to receive the plurality of pumpradiation beams from the first radiation source and the signal radiationbeam from the second radiation source and to emit a plurality of idlersin a second direction, wherein the plurality of idlers compriselow-noise copies of the signal.
 18. The method of claim 17, wherein anidler-generation noise figure is independent of a number of theplurality of pumps.
 19. The method of claim 17, wherein the firstradiation source comprises an optical frequency comb.
 20. The method ofclaim 17, wherein in the first radiation source comprises at least twoindividual lasers.